Exhaust emission control devices may comprise catalytic converters, evaporative emissions devices, scrubbing devices (e.g., hydrocarbon, sulfur, and the like), particulate filters, traps, absorbers, absorbers, reactors (e.g., non-thermal plasma reactors, and the like), and the like, as well as combinations comprising at least one of the foregoing devices. One function of these devices is to treat an exhaust stream, thereby reducing the concentration of at least one component in the exhaust stream. Such devices may be rated in terms of their performance, wherein the performance of an exhaust emission control device represents a measure of the ability of that device to reduce the concentration of a component(s) in a stream under various conditions.
Catalytic converters are one type of an exhaust emission control device, and comprise one or more catalytic materials deposited on a substrate. The composition of the catalytic materials, the composition of the substrate, and the method by which the catalytic material is deposited on the substrate serve as one way in which catalytic converters are differentiated from one another. Methods of depositing catalytic materials onto a substrate include washcoating, imbibing, impregnating, physisorbing, chemisorbing, precipitating, and combinations comprising at least one of the foregoing deposition methods.
Washcoating includes contacting an admixture of various components of a catalyst with a substrate such that a layer is deposited on and/or in the substrate. The term washcoat as used herein describes the layer or layers of the catalytically active admixture deposited on the substrate. In a three-way conversion catalyst, the washcoat may comprise a metal component, an aluminum oxide component, and an oxygen storage (OS) component. Stabilizers, promoters, binders, hydrogen sulfide control agents, and the like may also be present in a washcoat. Furthermore, the washcoat comprises layers of material.
These catalyst designs, the metal components, e.g., platinum group metals, are separated from each other in different catalyst layers and each layer in turn comprises support components that are optimized for the individual platinum group metals. For 2-layer designs, platinum (Pt) or palladium (Pd) may be located in the first layer in physical communication with the substrate (e.g., cordierite), while rhodium (Rh) is located in the top layer in physical communication with the exhaust stream. Generally, these 2-layer catalyst designs have the distinct disadvantage of being complex and are associated with a significant increase in manufacturing complexity and cost.
Therefore, what is need in the art is a 1-layer catalyst design with the performance of a 2-layer catalyst design.